1. Field of the Invention
The present invention relates to an organic electroluminescent device (OELD), and more particularly to a top emission type OELD having a high brightness.
2. Discussion of the Related Art
In general, an OELD emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. In comparison to a liquid crystal display (LCD) device, an additional light source is not necessary for the OELD to emit light, because the transition of the exciton between the two states causes light to be emitted. Accordingly, the size and weight of the OELD are less than the LCD device.
The OELD has other excellent characteristics, such as a low power consumption, superior brightness, and a fast response time. Thus, the OELD is seen as the display for the next-generation of consumer electronic applications such as cellular phones, car navigation systems (CNSs), personal digital assistants (PDAs), camcorders, palmtop computers, etc. Moreover, because fabricating the organic ELD is performed with fewer processing steps, the OELD is less expensive to produce than the LCD device.
In addition, the two types of OELDs are a passive matrix OELD and an active matrix OELD. While both of the passive and active matrix OELDs have a simple structure and are formed by a simple fabricating process, the passive matrix OELD requires a relatively large amount of power to operate. In addition, the display size of the passive matrix OELD is limited by the width and thickness of conductive lines used in the structure. Further, as the number of conductive lines increases, the aperture ratio of the passive matrix OELD decreases. In contrast, the active matrix OELDs are highly efficient and can produce a high-quality image on a large display with a relatively low power.
Turning now to FIG. 1, which is a schematic cross-sectional view of an OELD 1 according to the related art. As shown, the OELD 1 includes first and second substrates 12 and 28 facing and being spaced apart from each other. Also included is an array element layer 14 formed on the first substrate 12. As shown, the array element layer 14 includes a thin film transistor “T.” Although not shown, the array element layer 14 further includes a gate line, a data line crossing the gate line to define a pixel region “P,” and a power line crossing one of the gate and data lines. In addition, the OELD 1 also includes a first electrode 16 on the array element layer 14, an organic electroluminescent (EL) layer 18 on the first electrode 16, and a second electrode 20 on the organic EL layer 18. In addition, the first electrode 16 is connected to the thin film transistor “T.” Here, the organic EL layer 18 includes red (R), green (G) and blue (B) sub-organic EL layers in the pixel regions “P.”
In addition, the second substrate 28 functions as an encapsulating panel having a receded portion 21. A desiccant 22 is packaged in the receded portion 21 to protect the OELD 1 from moisture. Further, a seal pattern 26 is formed between the first and second substrates 12 and 28 in a periphery thereof so as to attach the first and second substrates 12 and 28 to each other.
Next, FIG. 2 is an equivalent circuit diagram of the related art ELD shown in FIG. 1. As shown in FIG. 2, a pixel region “P” is defined by a gate line 42 and a data line 44 crossing the gate line 42 formed on a substrate 32. Also included is a power line 55 spaced parallel from the gate line 42 and crossing the data line 44.
In addition, a switching element “TS” is connected to the gate and data lines 42 and 44 at an adjacent portion crossing the gate and data lines 42 and 44 and a driving element “TD” is connected to the switching element “TS.” For example, the driving element “TD” in FIG. 2 is a positive type thin film transistor. Further, a storage capacitor “CST” is formed between the switching element “Ts” and the driving element “TD.” Also, a drain electrode 63 of the driving element “TD” is connected to a first electrode (not shown) of an organic EL diode “E.” In addition, a source electrode 66 of the driving element “TD” is connected to the power line 55 and a gate electrode 68 is connected to the capacitor Cst and switching element Ts.
Hereinafter, an operation of the OELD will be explained in detail. When a gate signal is applied to the gate electrode 46 of the switching element “Ts,” a current signal applied to the data line 44 is changed into a voltage signal through the switching element “Ts” and is applied to the gate electrode 68 of the driving element “TD.”
Therefore, the driving element “TD” is driven and the level of the current applied to the organic EL diode “E” is determined such that the organic EL diode “E” can embody a gray scale. Further, because the signal in the storage capacitor “Cst” functions to maintain the signal of the gate electrode 68 of the driving element “TD,” the current applied to the EL diode is maintained until the next signal is applied even if the switching element “Ts” is in an OFF state.
Next, FIG. 3 is a schematic plan view of a related art OELD with respect to one pixel. As shown, the switching element “Ts,” the driving element “TD” connected to the switching element “Ts,” and the storage capacitor “Cst” are formed on the substrate 32 in the pixel region “P.” Alternatively, the switching element “Ts” and the driving element “TD” can be formed in multiple in the pixel region “P” in accordance with an operation characteristic thereof.
In addition, the substrate 32 includes a transparent insulating substrate such as glass or a plastic substrate. The gate line 42 is formed on the substrate 32 and the data line 44 crosses the gate line 42 to define the pixel region “P.” In addition, in this example, a power line 55 is parallel to the data line 44.
Further, the switching element “Ts” includes the gate electrode 46 connected to a first gate line 42, a first semiconductor layer 50 over the first gate electrode 46, a first source electrode 56 connected to the data line 44, and a first drain electrode 60 spaced apart from the first source electrode 56. The driving element “TD” includes the second gate electrode 68 connected to the drain electrode 60, a second semiconductor layer 62 over the second gate electrode 68, the second source electrode 66 connected to the power line 55, and the second drain electrode 63. Specifically, the first drain electrode 60 and the gate electrode 68 are connected to each other via a contact hole 64 of an insulating material layer (not shown).
Further, a first electrode 36 is connected to the first drain electrode 63 in the pixel region “P.” Although not shown, the storage capacitor “Cst” includes a first storage electrode of doped silicon, a second storage electrode occupying a portion of the power line 55, and an insulating material layer (not shown) between the first and second storage electrodes.
Turning now to FIG. 4, which is a schematic cross-sectional view of the related art OELD taken along the “IV-IV” line in FIG. 3. In FIG. 4, the second semiconductor layer 62 is formed on the substrate 32, a gate insulating layer “GI” is formed on the second semiconductor layer 62, the gate electrode 68 is formed on the gate insulating layer “GI” over the second semiconductor layer 62, and an interlayer insulating layer “IL” is formed on the gate electrode 68 and includes first and second contact holes “C1” and “C2” that expose both end portions of the second semiconductor layer 62. The source and drain electrodes 66 and 63 are formed on the interlayer insulating layer “IL” and are connected to the second semiconductor layer 62 via the first and second contact holes “C1” and “C2.”
A passivation layer 68 is also formed on the second source and drain electrodes 66 and 63 and includes a drain contact hole “C3” that exposes a portion of the drain electrode 63. The first electrode 36 is connected to the drain electrode 63 via the drain contact hole “C3,” the organic EL layer 38 is formed on the first electrode 36, and a second electrode 80 is formed on the organic EL layer 38. The first electrode 36, the organic EL layer 38, and the second electrode 80 constitute the organic EL diode “E.” Further, the driving element “TD” is a negative type TFT, and the first electrode 36 and the second electrode 80 are a cathode and an anode, respectively. Alternatively, the driving element “TD” is a positive type TFT, and the first electrode 36 and the second electrode 80 are an anode and a cathode, respectively.
In addition, the storage capacitor “Cst” and the driving element “TD” are disposed in a row. Here, the source electrode 66 is connected to the second storage electrode, and the first storage electrode 35 is disposed under the second storage electrode 34.
FIG. 5 is a schematic cross-sectional view of an emission region of the related art. In FIG. 5, the emission region of the OELD 1 includes the anode 36 on the substrate 32, a hole injection layer 38a on the anode 36, a hole transport layer 38b on the hole injection layer 38a, an emitting layer 38c on the hole transport layer 38b, an electron transport layer 38d on the emitting layer 38c, an electron injection layer 38e on the electron transport layer 38d, and the cathode 80 on the electron injection layer 38e. These layers are sequentially layered on the anode 36.
In addition, the hole transport layer 38b and the electron transport layer 38d function to transport a hole and electron to the emitting layer 38c to improve an emitting efficiency. Further, the hole injection layer 38c between the anode 36 and the hole transport layer 38b function to reduce a hole injecting energy, and the electron injection layer 38e between the cathode 80 and the electron transport layer 38d function to reduce an electron injecting energy, thereby increasing the emitting efficiency and reducing the driving voltage of the OELD.
Further, the cathode 80 is formed of a material including calcium (Ca), aluminum (Al), magnesium (Mg), silver (Ag) and lithium (Li). In addition, the anode 36 includes a transparent conductive material such as indium tin oxide (ITO). Thus, because the anode 36 formed with a transparent conductive material such as ITO is deposited by sputtering, layers under the anode 36 may be damaged. Therefore, to prevent damaging the emitting layer 38, the anode 36 is not formed on the emitting layer 38.
Accordingly, when light from the emitting layer 38 is emitted toward the anode 36 formed under the emitting layer 38, the substantial aperture region is limited due to the array element (not shown) under the anode 36. Consequently, because the OELD related art is a bottom emission type OELD, the brightness is deteriorated due to the array element. Further, to minimize the aperture region, the design of the array element is limited. Also, the driving element is selected from a positive type poly-silicon type in connection with the structure of the organic EL diode, the array process is complicated and the product yield is reduced.